Viscosity is a very important term in engineering and physics. We are using the term viscosity in general. Also, there are different types of viscosities. These different types are dynamic viscosity and kinematic viscosity. Here we explain the difference between kinematic viscosity and dynamic viscosity. Also, you can find other important terms about the viscosities.
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What is the Difference between Dynamic(Absolute) Viscosity and Kinematic Viscosity?
To understand the difference between them, we need to understand the term viscosity in general. Viscosity is the value that shows the internal resistance of a fluid to flow.
In fluid mechanics, we have a general experiment where we use two parallel plates. We are taking one plate at a speed of V and the other one is stationary. So, a velocity gradient of fluid occurs. The velocity of the fluid is V beside the moving plate, and the velocity of the fluid is 0 beside the stationary plate.
And also, we need to apply force to take that plate. Fluid beneath the plate will try to stop that plate. This shear force is because of the viscosity of the fluid. Fluid layers exert an opoosite resistance force to flow.
For Newtonian fluids, the shear force increases with the increasing deformation. Between the deformation and shear force, there is a linear relation. The most common fluids are Newtonian fluids such as water.
So, the facts that we stated above, the viscosity that we are mentioning is the dynamic viscosity. The unit of the dynamic viscosity is kg/m.s or lb/ft.s. Also, there is another unit to call minimal viscosities of common fluids. This unit is poise. Poise equals to 0.1 kg/m.s .For example, the absolute viscosity of the water is 1.002 centipoise. Centipoise is also 1/100 of the poise.
We can think of the dynamic viscosity as the stress-strain rate of fluids. Because, for the Newtonian fluids, there is a constant relation between the deformation and the shear stress. So, this constant relation is dynamic viscosity.
Also in heat transfer and thermodynamics analyses, we are using the kinematic viscosity value. If we divide the dynamic viscosity by kinematic viscosity, we obtain the kinematic viscosity value.
The unit of the kinematic viscosity is m/s2 or ft/s2.
Kinematic viscosity gaining importance for gases. Because there is a general relationship between the dynamic viscosity and density. But for fluids, there are no changes in densities in general.
The general difference between the kinematic viscosity and the dynamic viscosities of fluids is like above.
Force of a Fluid on a Plate
In addition, we can calculate the total force of flowing fluid on a plate with the equation below;
μ: This is the dynamic viscosity of the fluid. The unit of the dynamic viscosity is kg/m.s or lb/ft.s.
A: Area of the fluid that flows over a plate or surface. Unit os the area is m2 or ft2.
V: Velocity of the fluid. Unit is m/s or ft/s.
l: Layer thickness of the fluid flow which has the unit of m or ft.
So, we can understand from this equation, that with the increasing dynamic viscosity, the force increases. Also with the increasing area of the plate, the force increases. The same relation is valid between the velocity and the force.
But with the increasing layer thickness, the force decreases.
Non-newtonian fluids are also very important in fluid mechanics. For non-Newtonian fluids, there is no direct proportionality between the deformation and the stress.
For most of the non-Newtonian fluids, with the increasing deformation, the resistance to deformation increases. So, dynamic viscosities of Non-newtonian fluids change with the increasing deformation.
These types of fluids are generally called dilatants or shear thickening fluids. After a deformation value, they will act as a solid and they will elastically deform after a deformation.
Viscosity is a very important value for most engineering applications. For example, the pumping power to transmit a fluid for ranges is directly related to the viscosity. Because we need to apply a force to move this fluid. Viscosity is direct to resistance to this force.
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FAQs About Kinematic and Dynamic Viscosities
The difference between these viscosity values is, that we divide the dynamic viscosity by density to find kinematic viscosity. Kinematic viscosities are important for gases where their viscosities are changing with the changing density.
The unit of the dynamic viscosity is kg/m.s or lb/ft.s. And the unit of the kinematic viscosity is m.s2 or ft.s2. If you divide the dynamic viscosity value with density, you will find kinematic viscosity.
We are using the kinematic viscosity value in the thermodynamic and heat transfer calculations of gases. Because, with the changing pressures and temperatures, the densities of gases drastically change. With the changing density, the dynamic viscosity also changes. But in general, the relation between the dynamic viscosity and density has a constant relation. We use this constant relation which is the kinematic viscosity.
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